1 stick (8 tablespoons) unsalted butter, at room temperature
1/3 cup granulated sugar
1 extra-large egg, lightly beaten
1 teaspoon vanilla extract
1 1/2 cups all purpose gluten-free flour
3/4 teaspoon xanthan gum
1 1/2 teaspoons baking powder
1/4 teaspoon baking soda
1/4 teaspoon kosher salt
1/2 cup frozen blueberries
1/2 cup fresh or frozen cranberries
1 cup sour cream (low-fat is fine, nonfat is not)
1. Preheat your oven to 350 degrees F. Line a standard 12-cup muffin tin with liners and set aside the tin.
2. In a large bowl, beat the butter and sugar until light and fluffy and pale in color. Add the egg and vanilla, and beat until well-combined. Reserving a few tablespoons of the flour in a separate, small bowl, add the flour, xanthan gum, baking powder, baking soda, and salt, beating well after each addition. Toss the blueberries and cranberries in the reserved flour and set them aside. Add the sour cream to the mixture, and beat until well-combined.
3. After adding the sour cream, beat again vigorously for another 2 minutes or so, until the batter becomes thicker and a bit more elastic. That will mean that the xanthan gum has been activated. Add the berries to the batter, and mix until evenly distributed throughout. Divide the batter evenly among the 12 muffin cups. With wet hands, smooth the top of the muffins (we don’t want anything to burn).
4. Place the muffin tin in the center of the preheated oven and bake at 350 degrees F for 18-22 minutes, or until a toothpick inserted into the center of the muffins comes out mostly clean, or with only a few moist crumbs attached.
5. Allow the muffins to cool for at least 7 to 10 minutes in their wrappers, or the wrappers may be hard to remove. Serve warm or cool. These keep for days in an air tight container in the refrigerator.